There would be remnants for years!!!Sprinkle glitter everywhere
Change the Wi-Fi password (obviously only possible if they’ve left it as factory default and you can read it off the back, but surely most people do).Hide the router!
Evil!Oh thought of another one. This is fun! Playing the long game with this one ( depending on when in the year it is) tangle ALL Christmas lights in lots of knots if lights are on a prelit tree, take out the fuse bulb and several others hehe!